Model Studies of Near-Inertial Waves in Flow over the Oregon Continental Shelf

Abstract

Two-dimensional, primitive equation model studies of wind-forced flow over a continental shelf Show that, under upwelling conditions, high levels of near-inertial wave energy are found in the interior over the shelf. The regions of elevated wave energy, with maximum wave amplitudes of around ±0.2 m s?1, persist for up to two weeks and have spatial scales of 20?40 m vertically and 5?20 km horizontally. Relatively high dissipation levels are associated with these concentrations of wave energy. When forced with downwelling-favorable winds, model results show very little subsurface inertial energy on the shelf. A comparison of inertial wave propagation using the primitive equation model and a linearized version of the model demonstrates strong dependence on the background flow field. The behavior of inertial waves using the linearized model is completely different: very little subsurface inertial energy is seen on the shelf in either upwelling or downwelling conditions, except where the bottom slope is near the critical angle of reflection for incoming waves. In the primitive equation model, regions of elevated inertial wave energy occur where group velocities for near-inertial waves are reduced due to variations in the horizontal and vertical shear of the subinertial background flow. Critical angle reflection is a useful indicator of inertial energy concentration in the linearized model. In the primitive equation model, however, wave refraction by the spatially varying horizontal and vertical shear of the background flow complicates the interpretation of a near-bottom critical angle. The crucial factor governing the formation of wave energy concentrations seems to be the presence of an alongshore current with negative relative vorticity on the offshore side of the jet (southward jet on a west coast). Current meter measurements from the 1975 Winter-Spring Transition Experiment on the Oregon shelf show higher levels of subsurface inertial energy during upwelling than during downwelling, in agreement with the model results.